M.9 on certain soil types will not sustain the vigour needed for each combination of tree density.
Need a range of vigours (cont from last month)
M.9 on certain soil types will not sustain the vigour needed for each combination of tree density.
In Australia, a range of vigours will be needed for the different apple growing regions with different climates and soils, as M.9 vigour may be too weak for some soils.
A good understanding of the interactions between rootstocks and varieties, and climate and soil, would help growers select better rootstocks in the future.
Lessons from overseas
In some countries (e.g. Chile), there has been a significant trend towards high-density planting on M.9 rootstocks in newer blocks, but semi-dwarfing rootstocks such as MM.106 and M.7 are planted on new adjacent blocks.
In South Africa growers are worried that their trees on dwarfing rootstocks will be stunted in their relatively harsh summers, poor soils and restricted water supply.
Growers are also worried that trees will behave differently between apples on dwarfing rootstocks such as M.9, and their commonly used semi-dwarfing MM.106 rootstock. So growers are generally not confident to embrace dwarfing rootstocks for intensive plantings.
Consider resistance to pests and diseases
Growers must consider resistance to pests and diseases, such as woolly apple aphid and Phytophthora.
M.9 is not resistant to woolly apple aphid. Woolly apple aphid is much more a serious pest in the milder climates in Australia than in the northern hemisphere. The attack of woolly apple aphid on trees on M.9 is accentuated because the trees are already less vigorous.
Damaged root systems of young trees severely decrease the uptake of water and nutrients by the trees. Older and more vigorous trees are less affected, but contaminated fruit and unpleasant conditions for pickers and pruners are a problem.
With the introduction and use of effective pesticides against woolly apple aphid in the past, fruit growers have been game enough to use non-resistant rootstocks, such as M.9.
Withdrawal of, or restrictions on recent pesticides, may lead this pest to become economically significant once again.
With the slow resistance developing to certain chemicals and limited availability of appropriate chemicals, biological control is needed for woolly apple aphid.
Crop loads and fruit maturity
Excessive crop loads readily stall the growth of young trees on M.9.
Many growers overcrop their young trees at the expense of growth of trees, because the growers are anxious to get their orchard into production and have fruit to sell.
Generally, fruit on trees on dwarfing rootstocks, such as M.9, tend to ripen fruit earlier than more vigorous rootstocks. Although in some years the differences can be great, they cannot be predicted.
Fruit maturity of trees on MM.106 may be delayed for up to six days more than trees on M.9.
Crop load has a huge effect on time of maturity, and especially the rate that fruit colour develops. Early heavy thinning to optimal crop loads will advance maturity of fruit.
Nutrition and soils
Young apple orchards on M.9 need good nutrition for strong spring growth and good bud break, and do not compete well for water and nutrients.
With trees on M.9, light or sandy soils need to be mulched.
For best results, apple trees on M.9 need deep, fertile, and well-drained soils, and the best orchard management.
Where unfavourable climates and soils prevent tree growth extending further into the season, and to maximise the performance of high density apple orchards on M.9, growers may have to:
Ridge the shallow top soil
Install drip irrigation (preferably subsurface and pulsating)
Fertigate frequently
Mulch the soil
Cover the trees with net and/or install overhead cooling
Plant well-feathered trees closely enough so they can quickly fill their allotted spaces and reach their maximum permissible height.
What if these measures cannot be implemented or are not cost-effective? (cont next issue)
See this article in Tree Fruit July 2017
